1. Field of the Invention
The present invention relates to color ink sets having excellent color reproduction and lightfastness.
2. Background Art
In general, images produced by ink compositions, when exposed to indoor or outdoor light beams, the open air and the like for a long period of time, are frequently deteriorated due to an influence of ultraviolet light, visible light, various gases contained in the air or the like. In particular, in the case of color images produced by a plurality of ink compositions, the presence of even one color ink composition having poor lightfastness causes a change in hue of images, resulting in extremely deteriorated quality of color images. This has led to a demand for ink compositions capable of yielding highly weathering-resistant images which, even when stored for a long period of time, are less likely to be deteriorated.
Further, in recent years, attention has been drawn to ink jet recording. The ink jet recording is a printing method wherein droplets of an ink composition are ejected and deposited onto recording media, such as paper, to conduct printing. This method has a feature that images having high resolution and high quality can be printed at a high speed by means of relatively inexpensive apparatuses. Ink jet recording apparatuses utilizing this method are commercially widely accepted by virtue of good print quality, low cost, relatively quiet operation, and graphics-forming capability. Among others, thermal (bubble jet) and piezoelectric drop-on-demand printers have been very successful on the market, and have been widely used as printers for personal computers for office and domestic applications.
In recent years, a method has become used wherein a plurality of color ink compositions are provided and used in the formation of color images by ink jet recording. In general, color images are formed using three colors of a yellow ink composition, a magenta ink composition, and a cyan ink composition, and optionally four colors of the above three color compositions plus a black ink composition. In some cases, six colors of the above four colors plus a light cyan ink composition and a light magenta ink composition are used in the formation of color images. What is required of ink compositions for the formation of such color images is that they as such can exhibit good color development and, in addition, when used in combination with a plurality of ink compositions, can develop good intermediate colors.
Further, ink compositions constituting ink sets used in this ink jet recording are required to have properties such that they have suitable property values, such as proper viscosity and surface tension values, have high optical density, can yield sharp color tone and images, can yield images having excellent fastness properties, such as excellent waterfastness and lightfastness, have excellent storage stability, are less likely to cause nozzle clogging, and are free from problems associated with an odor and safety. In order to satisfy a large part of these property requirements, water-base inks prepared by dissolving water-soluble dyes in water or in a mixed solution of water and a water-soluble organic solvent are used. At that time, ink compositions having various hues are prepared from dyes having various hues.
Various image properties, such as color tone, lightfastness, and waterfastness, are greatly influenced by the properties of colorants in the ink compositions, and various dyes have hitherto been studied in the art.
The present inventors have now found that ink sets comprising a combination of specific color ink compositions can realize images having excellent color reproduction and, at the same time, having excellent lightfastness. The present invention has been made based on such finding.
Accordingly, it is an object of the present invention to provide an ink set which can realize images having excellent lightfastness and color reproduction.
Thus, according to one aspect of the present invention, there is provided a magenta ink set comprising two magenta ink compositions different from each other in color density, a magenta ink composition with higher color density and a magenta ink composition with lower color density,
said magenta ink composition with higher color density containing as a colorant a compound represented by formula (III) 
wherein
xcex1 represents a C1-C4 alkyl or alkoxy group or an OHxe2x80x94, SO3Hxe2x80x94, or COOM-substituted phenyl or naphthyl group;
xcex2 represents a hydrogen atom or a group represented by formula 
xe2x80x83wherein
R1represents a hydrogen atom or an OHxe2x80x94 or COOH-substituted C1-C4 alkyl group,
R2 represents an OHxe2x80x94, OCH3xe2x80x94, OC2H5xe2x80x94, SO3Mxe2x80x94, or COOM-substituted C1-C4 alkyl or phenyl group, and
R3 represents an OH, COOH, or NHR4 wherein R4 represents an SO3Mxe2x80x94 or COOH-substituted C1-C4 alkyl group; and
M represents a hydrogen atom, lithium, sodium, potassium, ammonium, or an organic amine,
said magenta ink composition with lower color density containing as a colorant a compound represented by formula (IV): 
wherein
M represents a hydrogen atom, lithium, sodium, potassium, ammonium, or an organic amine;
R5 represents a hydrogen atom, a chlorine atom or a group represented by formula 
xe2x80x83wherein
X represents an anilino group substituted by at least one SO3M and
Y represents OH, a chlorine atom, or a morpholino group; and
R6 represents a hydrogen atom, a chlorine atom, SO3M, or a C1-C4 alkyl group.
According to another aspect of the present invention, there is provided a magenta ink composition comprising as a colorant the compound represented by formula (III) and the compound represented by formula (IV) in a weight ratio of 1:1 to 1:5. Further, according to the present invention, there is provided an ink set comprising this magenta ink composition.
According to a further aspect of the present invention, there is provided an ink set comprising: a yellow ink composition; two magenta ink compositions different from each other in color density, a magenta ink composition with higher color density and a magenta ink composition with lower color density; and two cyan ink compositions different from each other in color density, a cyan ink composition with higher color density and a cyan ink composition with lower color density, said ink compositions containing as colorants the following respective specific compounds.
Specifically, the ink set according to this aspect of the present invention comprises: a yellow ink composition; two magenta ink compositions different from each other in color density, a magenta ink composition with higher color density and a magenta ink composition with lower color density; and two cyan ink compositions different from each other in color density, a cyan ink composition with higher color density and a cyan ink composition with lower color density,
said yellow ink composition containing as a colorant a compound represented by formula (I) and/or a compound represented by formula (II): 
wherein
R1, R1xe2x80x2, R2, and R2xe2x80x2 each independently represent CH3 or OCH3; and
A and Axe2x80x2 each independently represent a group having a structure selected from the group consisting of 
xe2x80x83wherein
M represents a hydrogen atom, lithium, sodium, potassium, ammonium, or an organic amine; and
n is an integer of 1 or 2,
said magenta ink composition with higher color density containing as a colorant a compound represented by formula (III): 
wherein
xcex1 represents a C1-C4 alkyl or alkoxy group or an OHxe2x80x94, SO3Hxe2x80x94, or COOM-substituted phenyl or naphthyl group;
xcex2 represents a hydrogen atom or a group represented by formula 
xe2x80x83wherein
R1 represents a hydrogen atom or an OHxe2x80x94 or COOH-substituted C1-C4 alkyl group,
R2 represents an OHxe2x80x94, OCH3xe2x80x94, OC2H,xe2x80x94, SO3Mxe2x80x94, or COOM-substituted C1-C4 alkyl or phenyl group, and
R3 represents an OH, COOH, or NHR4 wherein R4 represents an SO3Mxe2x80x94 or COOH-substituted C1-C4 alkyl group; and
M represents a hydrogen atom, lithium, sodium, potassium, ammonium, or an organic amine,
and/or a compound represented by formula (IV): 
wherein
M represents a hydrogen atom, lithium, sodium, potassium, ammonium, or an organic amine;
R5 represents a hydrogen atom, a chlorine atom or a group represented by formula 
xe2x80x83wherein
X represents an anilino group substituted by at least one SO3M and
Y represents OH, a chlorine atom, or a morpholino group; and
R6 represents a hydrogen atom, a chlorine atom, SO3M, or a C1-C4 alkyl group,
said magenta ink composition with lower color density containing as a colorant the compound represented by formula (III) and/or the compound represented by formula (IV),
said cyan ink composition with higher color density and said cyan ink composition with lower color density each containing as a colorant a compound represented by formula (V): 
wherein
R7 represents OH, COOM, or R8COOM wherein R8 represents a C4-C9 alkyl group; and
1, m, and n are each 0 to 4 with (1+m+n)=4.
According to the ink sets of the present invention, the presence of specific compounds as the colorant in the respective ink compositions can significantly improve the color reproduction of printed images and, at the same time, can yield images having improved lightfastness. Further, according to the ink sets of the present invention, nozzle clogging of a recording head can be prevented to realize stable ink ejection.